for a carseat on a plane, another question you need to ask yourself is how many carryons are you taking? I have seen several parent/small child tag teams getting on and off a plane with a backpack, a rollerboard and a carseat, trying to navigate the isle to get to their seats. it isn't pretty trying to make sure your little one is listening to you when you say walk to the row, having a rollerboard behind, and the car seat in front with a backpack on.
As a FF with my children (now 5 and 11) I never did the car seat on the plane, and the past 2 years I didn't take one at all. most rental agencies, in the US or abroad, can rent you one for a minimal cost, and considerable less hassle.
Same with strollers. I stopped taking the fancy one where the car seat fits in it, and got a simple umbrella stroller I could throw away at the end of the trip, just because the logistics of luggage, tired kids, tired momma and all of our items was just to much to deal with. small and simple for me.